Psychrometric Relation between Specific humidity, humidity ratio or moisture content

Psychrometric Relation:

Specific humidity, humidity ratio or moisture content:

It is the mass of water vapour present in 1 kg of dry air (in the air-vapour mixture) and is generally expressed in g/kg of dry air. It may also be defined as the ratio of mass of water vapour to the mass of dry air in a given volume of the air-vapour mixture.

Let

And

Assuming that the dry air and water vapour behave as perfect gases, we have for dry air,

And for water vapour

Also

also

From equation (1) and (2), we have

Substituting R_a = 0.287 kJ/kg K for dry air and R_v = 0.461 kJ/kg K for water vapour in the above equation, we have

For Saturated air (i.e. when the air is holding maximum amount of water vapour), the humidity ratio or maximum specific humidity,

.